The present disclosure relates generally to solid-state neutron detectors, and more specifically to the structure and fabrication of high sensitivity compact solid-state neutron detectors.
Low energy neutron (En<1 eV) detection plays an increasingly critical role in many applications including medicine, high energy physics (HEP), and home land security. Currently two inch helium-3 (3He) tube neutron detectors can achieve detection efficiencies of up to 80%, while portable handheld one inch 3He tube neutron detectors can achieve efficiencies of 10-15%. These efficiencies are reasonable for detection of thermal neutrons in high flux environments, such as in HEP applications, however they become impractical in low flux environments, such as in detecting presence of radioactive material for homeland security. Moreover, since the discovery of the scarcity of 3He, substitute neutron detectors have become a priority. 3He is a rare isotope of helium primarily obtained from radioactive decay of tritium, and since the end of the cold war the United States have consistently reduced their nuclear stockpile making the helium isotope scarce. Also, the demand of helium-3 isotopes have increased primarily for use in neutron detectors, and current supplies are not able to meet the demand.
One alternative approach has been the use of scintillator, water-based, and semiconductor type detectors. Thus far the major challenge with semiconductor detectors is the ability to fabricate very thick layers of semiconducting materials and fabricate efficient devices from them. Thick layers are typically required due to the low neutron capture cross sections of most available semiconductors. For example, the neutron capture cross-section of silicon-28 isotope is ˜1 barn (10−24 cm2), compared to boron-10 (10B), which is close to 3,840 barns. If for example, 10B is used as the converter material to a semiconductor type neutron detector, alpha particles (1.47 MeV) and lithium-7 (′Li) (0.84 MeV) particles are generated, which travel through the semiconductor and generate electron-hole pairs that are collected by the contacts. However, the absorption of these particles in the convertor material boron is very high with a mean free path of only 3.3 μm, which means they would be absorbed within boron, before getting to the semiconductor interface. If the thickness of the converter material is reduced, the neutron absorption drops resulting in lower efficiencies.
Currently used large area detectors for neutron detection in HEP testing facilities for research are facing a shortage in the source of helium-3. Moreover, the helium gas tubes are bulky mechanically and thermally unstable, and therefore pose several risks to developing portable neutron detectors. Alternative neutron technologies that employ 10B and boron trifloride (BF3) lined proportional detectors and lithium-6 (6Li) scintillators are promising, but are still bulky and require high voltages (>1000 V). Portable neutron detectors developed by PartTec manufacturing employ 6LiF:ZnS(μg) scintillators that emit blue light at 420 nm that is collected in wavelength shifting optical fibers and converted to 500 nm (green) photons still employ high sensitivity photon multiplier tubes (PMTs) which are bulky, mechanically and thermally unstable, and expensive. Moreover, the combination of fiber optic cables and PMTs make it difficult to realize handheld portable neutron detectors.
In contrast, semiconductor based devices can have a low operating voltage, small device footprint, and excellent stability. The dilemma is how to best utilize semiconductors to design an optimized thermal neutron detector. Most semiconductor materials have very low neutron capture cross-sections, the most common approach to fabricate solid state neutron detectors is to coat the semiconductor devices with materials that have higher capture cross-sections. Thus the neutron absorbing layer generates charged ions and ionizing radiation that are detectable by the semiconductor. The ions are only detectable if the mean free path of the charged particle is sufficient enough to reach the interface of the semiconductor/moderator. Thus conventional neutron sensitive thin-film coated semiconductors face the challenge of balancing the neutron absorption efficiency in the thin film and charge transfer efficiency to the semiconductor junction. Thin film semiconductor neutron detectors have low neutron capture efficiency, for example Mg2B14 thin films deposited on silicon diodes have an efficiency of only 1.3%.
In order to overcome this challenge, researchers have employed solid form semiconductor devices that are composed at least partially by a neutron sensitive material. Typically solid form semiconductors are grown by chemical vapor deposition (CVD) and have the neutron sensitive material embedded in the crystal of the semiconductor. However, these techniques are usually very expensive and not suitable for growing thick films.
One material recently developed by Stowe et. al. is a 6LiInSe2 that utilizes the 6Li capture cross-section of 938 barns as a bulk semiconductor. The material developed using the bridgeman method, has a bandgap of 2.85 eV and a high bulk resistivity of 3.17×1011 Ω-cm. The material is claimed to have very high neutron detection efficiencies compared to 3He tube detectors, primarily due to the close packed nature of the 6Li atoms in the crystal compared to 3He atoms in gaseous form. Thus utilizing materials with higher neutron capture cross-sections will only further improve the neutron detection efficiency and enable the development of handheld low cost and low power neutron detectors.
Recent developments on improvement of the growth quality of III-Nitride materials and the availability of GaN wafers, allow for the development of high efficiency harsh environment resistant devices. The group III-Nitride ternary compounds composed of AlxGa1-xN (bandgap 6.2 to 3.42 eV) and InxGa1-xN (bandgap 3.42 to 0.62 eV) exhibit inherent chemical and thermal ruggedness, which makes them suitable for several space and military applications. It has recently been determined that these Nitride materials can also offer exceptional radiation tolerance that is well beyond what can be achieved with conventional materials that are currently flown in space. For example silicon based detectors have shown up to 65% loss under irradiation of 1012 protons/cm2. III-Nitride materials present several advantages over other semiconducting materials currently used for optoelectronic devices. These include: 1) intrinsic radiation hardness; 2) ability to tune the bandgap from 6.2 eV to 0.62 eV; 3) layers of different bandgaps can been grown using well developed MBE or MOCVD methods; 4) large bandgap resulting in low thermal noise; 5) negative electron affinity (NEA) enabling field emission capabilities; 6) high degrees of chemical, mechanical, and thermal stability, plus high resistance to sputtering.
Gadolinium isotopes 155 and 157 have higher capture cross sections compared to any other materials, with 155Gd having a cross section of 65,000 barns and 157Gd having a cross section of 255,000 barns. Most important is that the mean free path of neutrons in 157Gd is only 1.3 μm, and a thickness of 6 μm is sufficient to stop 99% of thermal neutrons (25 meV).
GdN is a semiconductor with a half metallic bandgap calculated to be ˜0.6 eV, however, recent experimental evidence based on the optical absorption spectrum of rf magnetron sputter deposited GdN have shown bandgaps ranging from 1.03 eV to 0.95 eV. The lattice constant of bulk GdN (111) is about 4.9 Å, however, GdN (111) plane has a hexagonal symmetry that matches with the 0001 plane of InN with a lattice mismatch close to 0%. High quality growth of GdN (111) has been attempted on GaN and AlN films, however due to the lack of readily available InN substrates, to the best of our knowledge, it has not been attempted on InN.
Naturally occurring Gd has a neutron capture cross section of 49,700 barns, which is much higher than those of most materials. Compared to 6Li, the capture cross-section is more than 50 times higher for naturally occurring Gd, and it is more than 500 times higher if 157Gd is used. Of high interest is that Gd easily forms GdN, which is a semiconductor with an experimental bandgap of ˜1 eV. GdN (111) plane has a hexagonal symmetry that matches with the 0001 plane of III-Nitride materials. Several groups have grown GdN on GaN and MN, and have demonstrated polycrystalline quality GdN materials. FIG. 1 shows the alignment of the face centered cubic (fcc) structure of GdN with the hexagonal structure of GaN. The in-plane N—N interatomic distance is given by 4.9/√2=3.52 Å, where 4.9 Å is the lattice constant of GdN, this leads to a biaxial strain of −9.4%, that eventually leads to polycrystallinity.
In contrast, the InN lattice constant is 3.54 Å, making InN a perfect substrate match for GdN. However, due to the lack of readily available InN substrates and/or “templates”, there has been no work demonstrating the potential growth of GdN on InN.
Patent publication WO2013032549 describes a portable thermal neutron detector based on an array of Si CMOS transistors covered with a Gd containing film, such as gadolinium oxide, deposited by plasma enhanced atomic layer deposition.
Another patent publication, WO2011002906, describes at least thermal neutron detection with a capacitor type solid-state device based on gadolinium oxide deposited on low resistivity semiconductor substrate.
Patent publication WO2006085307 describes a solid-state device for detection of neutron and alpha particles detector that has an active region formed of a polycrystalline semiconductor compound containing 10Boron, 6Lithium, 113Cadmium, 157Gadolinium and 199Mercury. The semiconductor compound is sandwiched between an electrode assembly by using an organic or inorganic binder.
Patent publication WO2009117477 describes a silicon-on-insulator (SOI) neutron detector device with lateral carrier transport and collection detector structure within the active semiconductor layer of the silicon-on-insulator structure, and a neutron to high energy particle converter layer on the active semiconductor layer that includes cadmium, gadolinium, gadolinium phosphate, gadolinium oxide, and their combinations.
Patent publication WO2007030156 describes a device for neutron detection having semiconductor-based elements synthesized and used in the form of semiconductor dots, wires, or pillars on or in a semiconductor substrate embedded with matrixes of high cross-section neutron converter materials that can emit charged particles upon interaction with neutrons. These charged particles in turn can generate electron-hole pairs and thus detectable electrical current and voltage in the semiconductor elements.
High efficiency neutron detection can be achieved by using a design described in the patent publication WO2004040332. The detector utilizes a semiconductor wafer with a matrix of spaced cavities filled with one or more types of neutron reactive material such as 10B or 6LiF. The cavities are etched into both the front and back surfaces of the device such that the cavities from one side surround the cavities from the other side. The cavities may be etched via holes or etched slots or trenches. The cavities can be also different-sized and the smaller cavities extend into the wafer from the lower surfaces of the larger cavities. In one of the other embodiments multiple layers of different neutron-responsive material are formed on one or more sides of the wafer.
Publication number WO2010011859 discloses a room temperature operating solid state hand held neutron detector that integrates one or more relatively thin layers of a high neutron interaction cross-section element or materials with semiconductor detectors. The high neutron interaction cross-section element (e.g., Gd, B, or Li) or materials comprising at least one high neutron interaction cross-section element can be in the form of unstructured layers or micro- or nano-structured arrays. Such architecture provides high efficiency neutron detector devices by capturing substantially more carriers produced from high energy a-particles or Y-photons generated by neutron interaction.
Publication number US 20130075848 describes a three-dimensional boron particle loaded thermal neutron detectors utilizing neutron sensitive conversion materials in the form of nano-powders and micro-sized particles, as opposed to thin films, suspensions, paraffin, etc. More specifically, methods to infiltrate, intersperse, and embed the neutron nano-powders to form two dimensional and/or three-dimensional charge sensitive platforms are specified. The use of nano-powders enables conformal contact with the entire charge-collecting structure regardless of its shape or configuration.
Publications EP2494375 and WO2011051300 describe a device for detecting neutrons that comprises a neutron reactive material, a semiconductor element being coupled with the neutron reactive material, and electrodes are arranged for collecting the electrical charges and to provide electrically readable signal. The thickness of the first semiconductor element is so low that it is transparent for incident photons, such as background gamma photons. Pulsed laser deposition (PLD) is mentioned as one of the preferable methods for deposition of the neutron reactive material on the semiconductor surface.
There remains a need for solid-state neutron detectors that are highly efficient, compact, producible at low cost, and capable of alleviating decreased neutron detection efficiency in harsh environmental conditions.